NEET-PG 2012 Practice Questions

Q: Km value is defined as:

Substrate concentration at Vmax/2. ***Substrate concentration at Vmax/2*** - The **Michaelis constant (Km)** is defined as the **substrate concentration** at which the reaction velocity is **half of the maximum velocity (Vmax/2)**. - It reflects the **affinity of an enzyme for its substrate**; a lower Km indicates higher affinity. *Substrate concentration at which reaction rate is maximum* - The **maximum reaction rate (Vmax)** is achieved when the enzyme is **saturated with substrate**, meaning all active sites are occupied. - Km specifically refers to the substrate concentration needed to reach **half of this maximum rate**, not the maximum rate itself. *Substrate concentration at Vmax* - At **Vmax**, the enzyme is fully saturated with substrate, and the reaction rate cannot increase further by adding more substrate. - The **Km value** is a measure related to the **efficiency of substrate binding** at conditions below saturation, specifically at half Vmax. *Substrate concentration at which enzyme activity is optimal* - **Optimal enzyme activity** is generally influenced by factors such as **pH and temperature**, which affect the enzyme's structure and catalytic efficiency. - Km is specifically related to the **substrate concentration** required to achieve a specific reaction rate, not the overall optimal environmental conditions for the enzyme.

Q: How do enzymes function in biochemical reactions?

Decrease in activation energy. ***Decrease in activation energy*** - Enzymes act as **biological catalysts** by providing an alternative reaction pathway with a lower **transition state energy**. - This reduction in the **activation energy** allows a higher proportion of reactant molecules to overcome the energy barrier and react, thereby increasing the reaction rate. *Increase in activation energy* - This statement is incorrect as increasing activation energy would slow down the reaction rate, which is contrary to the function of enzymes. - Enzymes are designed to accelerate reactions, not inhibit them, by making them energetically more favorable to proceed. *Shift equilibrium constant* - Enzymes catalyze both the forward and reverse reactions equally, meaning they accelerate the rate at which equilibrium is reached but **do not alter the equilibrium constant (Keq)** of a reaction. - The equilibrium constant is determined by the difference in free energy between reactants and products, which enzymes do not change. *Provide energy to the reaction* - This statement is incorrect because enzymes do **not provide energy** to reactions; they only lower the activation energy barrier. - Enzymes facilitate reactions by stabilizing the transition state, not by adding energy to the system, which would violate thermodynamic principles.

Q: Enzymes that move a molecular group from one molecule to another are known as -

Transferases. ***Transferases*** - **Transferases** are a class of enzymes that catalyze the transfer of a specific functional group (e.g., methyl, acetyl, phosphate) from one molecule (the donor) to another (the acceptor). - This broad category includes enzymes vital for many metabolic pathways, such as **kinases** (transferring phosphate groups) and **transaminases** (transferring amino groups). *Ligases* - **Ligases** are enzymes responsible for joining two large molecules together, typically by forming a new chemical bond. - This process usually involves the concomitant hydrolysis of a small, energy-rich molecule such as **ATP**, to provide the necessary energy for bond formation. *Dipeptidases* - **Dipeptidases** are a type of hydrolase enzyme that specifically cleaves the peptide bond within a **dipeptide**, releasing two free amino acids. - They are crucial for the final stages of protein digestion, breaking down small peptides into absorbable **amino acid units**. *Oxido-reductases* - **Oxido-reductases** are enzymes that catalyze **oxidation-reduction reactions** (redox reactions), where electrons are transferred from one molecule to another. - This class includes enzymes like **dehydrogenases** and **oxidases**, which play critical roles in cellular respiration and energy production.

Q: Which sensory modalities are most directly affected by lesions of the primary somatosensory cortex?

Localization and two-point discrimination. ***Localization and two-point discrimination*** - Lesions in the **primary somatosensory cortex** (S1) lead to profoundly impaired **discriminative touch**, which includes the ability to precisely localize tactile stimuli and distinguish between two closely spaced points. - These are the **most characteristic deficits** of S1 lesions, representing the cortex's unique role in processing **spatial discrimination and fine sensory analysis**. - S1 is essential for the **integrative functions** that allow precise spatial mapping of sensory inputs. *Pain, temperature, and touch* - Basic touch perception is affected, but **pain and temperature** are primarily mediated by the **spinothalamic tracts** with substantial processing in the thalamus, insular cortex, and anterior cingulate cortex rather than S1. - Crude touch sensation remains relatively preserved with S1 lesions; it is the **discriminative quality** that is lost. - These modalities are NOT the most directly affected by isolated S1 lesions. *Vibration and proprioception* - **Vibration** and **proprioception** are indeed significantly impacted by S1 lesions as S1 receives thalamic projections from the **dorsal column-medial lemniscus (DCML) pathway**. - However, these modalities have substantial **subcortical representation** in the thalamus and can be partially preserved even with cortical damage. - In contrast, **localization and two-point discrimination** are purely cortical functions with no subcortical processing, making them the MOST directly and exclusively dependent on S1 integrity. *All of the options* - This is incorrect because pain and temperature perception is NOT most directly affected by S1 lesions—these are primarily processed by other pathways and cortical areas (spinothalamic system, insular cortex).

Q: Which of the following contains the PRIMARY central chemoreceptors responsible for detecting CO2 and pH changes in cerebrospinal fluid?

Medulla. ***Medulla*** - The **medulla oblongata** in the brainstem houses the primary central chemoreceptors. - These chemoreceptors are located on the **ventral surface of the medulla** and are highly sensitive to changes in the **pH of the cerebrospinal fluid (CSF)**, which is indirectly affected by the partial pressure of carbon dioxide (PCO2) in arterial blood. - CO2 diffuses across the blood-brain barrier, combines with water to form H+ ions, which directly stimulate these central chemoreceptors. *Baroreceptors in carotid sinus* - **Baroreceptors** primarily detect changes in **arterial blood pressure**, not CO2 or pH levels. - They are located in the carotid sinus and aortic arch and are involved in cardiovascular reflexes, not direct chemoreception for respiratory drive. *Peripheral chemoreceptors in carotid bodies* - **Peripheral chemoreceptors** in the carotid bodies (and aortic bodies) detect changes in **arterial blood O2, CO2, and pH**. - However, they are **peripheral**, not central chemoreceptors, and are the primary detectors of **hypoxemia**. - They contribute to respiratory drive but are secondary to central chemoreceptors for CO2 detection. *All of the above* - This option is incorrect because only the **medulla** contains the primary central chemoreceptors for CO2 and pH detection in CSF. - Baroreceptors detect blood pressure, and peripheral chemoreceptors are not central chemoreceptors.

Q: What is the normal range for the CSF/plasma glucose ratio?

0.6 - 0.8. ***Correct: 0.6 - 0.8*** - This ratio indicates that cerebrospinal fluid (CSF) glucose concentration is typically 60-80% of plasma glucose concentration - This range is crucial for identifying metabolic or infectious pathologies affecting the central nervous system - Normal CSF glucose is approximately 50-80 mg/dL when plasma glucose is 70-120 mg/dL *Incorrect: 0.2 - 0.4* - A ratio in this range indicates significantly low CSF glucose, suggesting conditions like bacterial meningitis or hypoglycorrhachia - This is well below the normal physiological proportion of glucose in the CSF relative to plasma - Seen in bacterial/tuberculous meningitis, fungal infections, or malignancy *Incorrect: 1.0 - 1.2* - A CSF/plasma glucose ratio close to or above 1.0 would imply that CSF glucose levels are equal to or higher than plasma levels, which is physiologically impossible under normal conditions - Glucose transport into the CSF is regulated by GLUT-1 transporters and typically results in lower concentrations than in plasma - The blood-brain barrier maintains this gradient *Incorrect: 1.2 - 1.6* - This range is even more exaggerated and physiologically impossible, as CSF glucose cannot exceed plasma glucose in a healthy individual - Such a high ratio would contradict the mechanisms of glucose transport across the blood-brain barrier - Would suggest laboratory error if observed

Q: Which of the following statements about volume receptors is NOT true?

They are located in carotid sinus. ***They are located in carotid sinus*** - Volume receptors, primarily **atrial stretch receptors** and receptors in the **pulmonary vessels**, are located in the low-pressure areas of the circulation, not the carotid sinus. - The carotid sinus primarily contains **baroreceptors** which detect changes in arterial pressure, not blood volume. *They are low pressure receptors* - This statement is true; volume receptors are indeed **low-pressure receptors** found in the atria and great veins. - They primarily monitor **extracellular fluid volume** and central venous pressure. *They provide afferents for thirst control* - This statement is true; when blood volume decreases, the firing rate of these receptors decreases, signaling the **central nervous system** to stimulate thirst. - This is an important mechanism for regulating **fluid intake** and maintaining hydration. *They mediate vasopressin release* - This statement is true; a decrease in blood volume reduces the afferent signaling from volume receptors, which consequently stimulates the release of **vasopressin (ADH)**. - Vasopressin then increases **water reabsorption** in the kidneys to conserve fluid.

Q: Wolff–Chaikoff effect is due to?

Excess iodine intake. ***Excess iodine intake*** - The **Wolff-Chaikoff effect** is a phenomenon where a high intake of iodine acutely **inhibits thyroid hormone synthesis** and release. - This effect protects the body from excessive thyroid hormone production during periods of very high iodine availability. *Decreased iodination of MIT* - While the Wolff-Chaikoff effect does inhibit **iodination**, the direct cause is the excessive iodine itself, which triggers an autoregulatory shutdown. - Decreased iodination is a *consequence* of the high iodine leading to inhibition of thyroid peroxidase activity, but not the primary cause of the effect. *Suppression of TSH secretion* - **TSH (Thyroid Stimulating Hormone)** secretion is primarily regulated by negative feedback from thyroid hormones (T3 and T4) and TRH from the hypothalamus. - The Wolff-Chaikoff effect directly involves the thyroid gland's response to iodine and is not primarily mediated by TSH suppression. *Decreased conversion of T4 to T3* - The **conversion of T4 to T3** primarily occurs in peripheral tissues, mediated by deiodinase enzymes. - The Wolff-Chaikoff effect focuses on the inhibition of **iodine organification** and hormone release within the thyroid gland itself, not peripheral conversion.

Q: Prostaglandins (PGs) in semen are secreted by?

Seminal vesicle. ***Seminal vesicle*** - The **seminal vesicles** are the primary source of **prostaglandins (PGs)** in semen, contributing significantly to the seminal fluid volume. - These PGs play a crucial role in promoting **sperm motility** and facilitating fertilization. *Prostate* - The **prostate gland** primarily secretes **citrate**, **acid phosphatase**, and **prostate-specific antigen (PSA)**, which contribute to sperm activation and semen liquefaction. - It does not significantly contribute to the prostaglandin content of semen. *Sperms* - **Spermatozoa** themselves primarily contribute genetic material and are not a significant source of prostaglandin synthesis in semen. - Their main function is fertilization, not the production of accessory gland secretions. *Testes* - The **testes** are responsible for **spermatogenesis** (sperm production) and the synthesis of **androgens** like testosterone. - They do not secrete prostaglandins into the seminal fluid.

Q: Growth hormone level is highest during

Sleep. ***Sleep*** - Growth hormone (GH) secretion is **pulsatile**, with the largest and most consistent pulses occurring during **slow-wave sleep** (deep sleep). - This nocturnal surge contributes significantly to the overall daily GH output and is crucial for growth and metabolic regulation. *Hypoglycemia* - While **hypoglycemia** is a potent stimulus for GH release, it is an acute stress response rather than a state where GH levels are consistently highest. - The body's primary response to hypoglycemia is to raise blood glucose, and while GH helps, it is not the peak physiological secretion time. *Fasting* - **Prolonged fasting** can increase GH secretion as a mechanism to mobilize fat stores and conserve glucose. - However, the peak levels due to fasting are generally less pronounced than the dramatic surge observed during deep sleep. *Exercise* - **Vigorous exercise** can acutely stimulate GH release, particularly with sustained effort. - This increase is typically transient and not as high or consistently cyclical as the secretion during nocturnal sleep.

Question 1

Km value is defined as:

Accepted Answer

Substrate concentration at Vmax/2

Answer

Substrate concentration at which reaction rate is maximum

Answer

Substrate concentration at Vmax

Answer

Substrate concentration at which enzyme activity is optimal

Question 2

How do enzymes function in biochemical reactions?

Accepted Answer

Decrease in activation energy

Answer

Increase in activation energy

Answer

Shift equilibrium constant

Answer

Provide energy to the reaction

Question 3

Enzymes that move a molecular group from one molecule to another are known as -

Accepted Answer

Transferases

Answer

Ligases

Answer

Dipeptidases

Answer

Oxido-reductases

Question 4

Which sensory modalities are most directly affected by lesions of the primary somatosensory cortex?

Accepted Answer

Localization and two-point discrimination

Answer

Pain, temperature, and touch

Answer

Vibration and proprioception

Answer

All of the options

Question 5

Which of the following contains the PRIMARY central chemoreceptors responsible for detecting CO2 and pH changes in cerebrospinal fluid?

Accepted Answer

Medulla

Answer

Baroreceptors in carotid sinus

Answer

Peripheral chemoreceptors in carotid bodies

Answer

All of the above

Question 6

What is the normal range for the CSF/plasma glucose ratio?

Accepted Answer

0.6 - 0.8

Answer

1.2 - 1.6

Answer

0.2 - 0.4

Answer

1.0 - 1.2

Question 7

Which of the following statements about volume receptors is NOT true?

Accepted Answer

They are located in carotid sinus

Answer

They are low pressure receptors

Answer

They mediate vasopressin release

Answer

They provide afferents for thirst control

Question 8

Wolff–Chaikoff effect is due to?

Accepted Answer

Excess iodine intake

Answer

Decreased iodination of MIT

Answer

Suppression of TSH secretion

Answer

Decreased conversion of T4 to T3

Question 9

Prostaglandins (PGs) in semen are secreted by?

Accepted Answer

Seminal vesicle

Answer

Prostate

Answer

Sperms

Answer

Testes

Question 10

Growth hormone level is highest during

Accepted Answer

Sleep

Answer

Hypoglycemia

Answer

Fasting

Answer

Exercise

NEET-PG 2012

Biochemistry

Physiology